Appartuses and processes for producing optical effect layers comprising oriented non-spherical magnetic or magnetizable pigment particles

Abstract

The present invention relates to the field of magnetic assemblies and processes for producing optical effect layers (OEL) comprising magnetically oriented non-spherical magnetic or magnetizable pigment particles on a substrate. In particular, the present invention relates magnetic assemblies and processes for producing said OELs as anti-counterfeit means on security documents or security articles or for decorative purposes.

The invention claimed is: 
     
       1. A process for producing an optical effect layer on a substrate, said process comprising the steps of:
 i) applying on a substrate surface a radiation curable coating composition comprising non-spherical magnetic or magnetizable pigment particles, said radiation curable coating composition being in a first state, 
 ii) exposing the radiation curable coating composition to a magnetic field of an apparatus comprising:
 a) a magnetic assembly comprising a non-magnetic supporting matrix and:
 a1) a loop-shaped magnetic-field generating device being either a single loop-shaped dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or a combination of two or more dipole magnets disposed in a loop-shaped arrangement, each of the two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction, and 
 a2) a single dipole magnet having a magnetic axis substantially perpendicular to the substrate surface or two or more dipole magnets having a magnetic axis substantially perpendicular to the substrate surface and having a same magnetic field direction and/or one or more pole pieces, 
 
 b) a magnetic-field generating device being either a single bar dipole magnet having a magnetic axis substantially parallel to the substrate surface or a combination of two or more bar dipole magnets, each of the two or more bar dipole magnets having a magnetic axis substantially parallel to the substrate surface and having a same magnetic field direction, 
 
 so as to orient at least a part of the non-spherical magnetic or magnetizable pigment particles, and 
 iii) at least partially curing the radiation curable coating composition of step ii) to a second state so as to fix the non-spherical magnetic or magnetizable pigment particles in their adopted positions and orientations, 
 wherein the optical effect layer provides an optical impression of one or more loop-shaped bodies having a size that varies upon tilting the optical effect layer, 
 wherein the magnetic assembly and the magnetic-field generating device are arranged one on top of the other and are in direct contact, 
 wherein the loop-shaped magnetic-field generating device and the single dipole magnet or the two or more dipole magnets of the magnetic assembly are disposed within the non-magnetic supporting matrix. 
 
     
     
       2. The process according to  claim 1 , wherein the magnetic assembly comprises the supporting matrix and:
 a1) the loop-shaped magnetic-field generating device, and 
 a2) the single dipole magnet or the two or more dipole magnets and the one or more pole pieces. 
 
     
     
       3. The process according to  claim 1 , wherein at least a part of the plurality of non-spherical magnetic or magnetizable particles is constituted by non-spherical optically variable magnetic or magnetizable pigment particles. 
     
     
       4. The process according to  claim 3 , wherein the optically variable magnetic or magnetizable pigments are selected from the group consisting of magnetic thin-film interference pigments, magnetic cholesteric liquid crystal pigments and mixtures thereof. 
     
     
       5. The process according to  claim 1 , wherein step iii) is carried out partially simultaneously with the step ii). 
     
     
       6. The process according to  claim 1 , wherein the non-spherical magnetic or magnetizable particles are platelet-shaped pigment particles, and wherein said process further comprises a step of exposing the radiation curable coating composition to a dynamic magnetic field of a first magnetic-field-generating device so as to bi-axially orient at least a part of the platelet-shaped magnetic or magnetizable pigment particles, said step being carried out after step i) and before step ii). 
     
     
       7. An optical effect layer produced by the process recited in  claim 1 . 
     
     
       8. A security document or a decorative element or object comprising one or more optical effect layers recited in  claim 7 . 
     
     
       9. The process according to  claim 1 , wherein the radiation curable coating composition on the substrate is exposed to the magnetic field of the apparatus above one side of the magnetic assembly and the magnetic-field generating device is arranged on an opposite side of the magnetic assembly. 
     
     
       10. The process according to  claim 1 , wherein the optical effect layer provides an optical impression of one or more loop-shaped bodies having a size that varies upon tilting the optical effect layer and the one or more loop-shaped bodies appear to continually vary in size as an angle at which the optical effect layer is tilted changes. 
     
     
       11. The process according to  claim 1 , wherein step i) is carried out by a printing process. 
     
     
       12. The process according to  claim 11 , wherein the printing process is selected from the group consisting of screen printing, rotogravure printing and flexography printing. 
     
     
       13. The process according to  claim 1 , wherein the optical effect layer provides an optical impression of one or more loop-shaped bodies having a size that continually increases or continually decreases as the optical effect layer is tilted from a negative viewing angle to a positive viewing angle.